298 Rolls for bending Section Bars. Vertical rolls are often used for long, heavy plates, and are said to be less expensive in operation, while giving truer finish to the end of the bent plate. This last is the principal difficulty with all rolls, the entering edge, to six inches deep, being always set by bending while hot with wooden hammers. Except for this, the plates are never heated for rolling, even up to i\ inches in thickness, for in such cases the radius is proportionately larger. The weight of plate is eliminated by the vertical method, with less fear of obliquity of curvature. Long rolls are often slightly bellied at the centre, to take up spring. For the heavier plates an hydraulic bender, introduced by Mr. Tweddell, seems very likely to super- sede rolls. It finishes the plates to a truer circle from end to end, and there is no limit to plate thickness, or risk of fracture by too rapid feed. Butt strips can also be bent truly to boiler curve. The tool is similar in design to the multiple punch in Fig. 2840, but the girders are placed vertically, and suitable dies inserted instead of the row of punches. Plate-straightening rolls are similar in construction, but there are some four rollers at top, pressed down simultaneously by connected screws, upon three rollers at bottom, and the plate is passed through and through till truly plane. Rolls for Section Bars (Fig. 288) have their axes vertical, and are placed upon a table A, which is sometimes conveniently set flush with the ground, with a pit for the gearing. They are driven by the usual fast and loose pullies F and L with crossed and open straps for reversal, These actuate a worm and worm wheel, B and c, and a spur pinion D on the axis of c gears with wheels G G on the roller shafts. Thus BE are the driving rollers, and H the bending roller, with a screw j to bring its bearing closer to the rollers E E, effected by turning the nut K. A ring or angle bar is shewn bent to a circle with an outward flange—an inward- flanged ring being obtained by turning all the rollers upside down, and other sections by special rollers. Finally the ring is removed and welded with a glut-piece. V Flanging Presses.—It being always advisable to dimmish the number of joints in a boiler, the end plate is usually flanged or bent over at the edge to form a ledge for the shell-plate, while stiffening itself considerably.